I just finished reading this article and all I can say is wow. Extremely well written and very interesting. I will read it again to make sure I am taking in all of the information because it is packed with more info than my humble brain can digest in one reading. My current refractor is a 101mm f5.4 APO. I have been thinking about a larger refractor and only gave cosideration to another APO. This article has caused me to consider a long focus achro as a possibility.

I take my hat off to Neil and his valued co-researcher. Being only a layman with a passion for visual astronomy and traditional (and some new) refractors, this is a clearly well researched, well written and very informative piece of work.

It deserves serious study and constructive responses from those academics with a far deeper understanding of the science of optics than I have.

For my own part, I am content to be pleased that my "gut feeling" about long focus refractors being in some way better able to present stable, clear images compared to their very short focus new exotic counterparts, appears to be founded in good science.

Neil, you deserve some kind of public recognition for this piece of work, as does your colleague. Well done!

Truly a modern "magnum opus" and must read for anyone owning or considering a refractor.

Neil,

An exceptional effort for sure! It will stand many enjoyable readings and re-readings ... a wealth of both finidngs and implications that I'm sure will bear much fruit. Destined to be a classic reference for the community in the years to come. Hats off to Vlad for the outstanding technical support as well. Quite deserves the coining of the "Sacek Effect" !!

That was the sound of two or more decades of misconceptions shrieking as they are thrown out of the window.

Truly a modern "magnum opus" and must read for anyone owning or considering a refractor.

Exceptional and well done!

Regards,

Jim

And I thought it was a jet!

Yes, very well written and, I might add, very even-handed in it's treatment of achromats and APO's. Neil has established a very solid foundation for future research - although, I'm not sure at this point what else can be added.

Excellent article, very well written, and makes the complex science very understandable. I have always had a suspicion that this might be the case, but now there is solid science to explain it. Well done!

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy... and how horse drawn transportation is actually far better than motorized vehicles.

I've used long focus achromats and compared them to all manner of doublet and triplet apo optics.

IMO, long achros show less sensitivity to seeing conditions, but that's the only inherent advantage they offer.

If somebody has a vested interest in championing the alleged superiority of a particular design, then it's not surprising that they will continue to promote it. Present lots of convincing looking charts, graphs and statistics, which apparently prove their point.

Can somebody point me to the 4" f/6.3 doublet apo mentioned in this article? I'd like to *actually* compare it to my 4" f/15 achromat.

Then again, I have observed with a 4.3" f/5.95 ED doublet stopped down to 4" F/6.3 and can report that it outperforms the 4" f/15 achro in all respects, except for sensitivity to seeing.

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy...

As profession drag racer Warren Johnson characterized it, properly tuned, carburetors make more peak power than EFI in a Pro Stock engine. A carb’s pressure differential atomizes the gas a lot better than spraying fuel through an orifice. And as professional racer Myron Cottrell puts it, if you're looking to drive the family car about 25k miles a year, get EFI; if you are looking to race, get a carb. With every advantage, comes a disadvantage...but this is off topic

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy...

As profession drag racer Warren Johnson characterized it, properly tuned, carburetors make more peak power than EFI in a Pro Stock engine. A carb’s pressure differential atomizes the gas a lot better than spraying fuel through an orifice. And as professional racer Myron Cottrell puts it, if you're looking to drive the family car about 25k miles a year, get EFI; if you are looking to race, get a carb. With every advantage, comes a disadvantage...but this is off topic

Moral of the story: Pick your analogies carefully, CN members have diverse and varied experience

Whilst I agree that long focus achromats can be excellent telescopes, one of the main reasons for buying a shorter focal length Apochromat is because of the optical quality, combined with its shorter, more manageable length. This would probably outweigh any slight optical differences for many owners. When I decided to go to a bigger refractor, I chose the 178ed because a similar achro ( probably a DG "8 f15), would have meant going round to knock on my neighbours door every time I wanted to remove the dew shield. I ask, in all humility,that this discovery be called: "the not waking up the neighbour at two in the morning effect".

Thank you all for the fantastic feedback this article has so far generated. I am overwhelmed, really!

Some folk have kindly contacted me to let me know of a few little errors that have inadvertently crept into the text. Mea culpa! Hope to rectify them soon.

Jim: hope you didn't choke on your roasted chicory

Clive: I need to emphasise that the purpose of the article is to evaluate the performance of fast apos (can I call them that?) and long focus achromats primarily with respect to their perceived image stability. The intention was not to be adversarial but to point out how the respective instruments are likley to perform in the field. There are 4" F/6 doublets already in existence, so F/6.3 was not a fictional scenario in any way, shape or form.

Bill: The Sacek effect is something we all need to talk about sometime soon. It is fascinating to say the least!

Banatop:

"I ask, in all humility,that this discovery be called: "the not waking up the neighbour at two in the morning effect".

Neil has done a great job producing his report.
He's a masterful writer.
It obviously involved a lot of research on his part, with the assistance of Vlad.
Lots of interesting questions were raised and explored.

It just doesn't jibe with my experiences, which is why I have a problem with some of the findings.
Mathematical modeling, graphs and spot diagrams sometimes don't accurately describe what's going on. Or, they do but context is misinterpreted.

Case in point--
Some enthusiasts of a small, very well made Maksutov-Cass telescope are known to extoll it's virtues with great exuberance. That instrument's central obstruction has been argued (along with proof derived from optical modeling) to increase the scope's resolving power, compared to an unobstructed system. However, in real-world usage, it's obstructed optic doesn't perceptibly improve resolution and in the vast majority of situations, image quality is degraded. But that never seems to inhibit the diehard enthusiasts from revisiting the "central obstruction is good" viewpoint.

So, around and around we go.

Best wishes to Neil.
He is a great ambassador of the long focus achromat.

If somebody has a vested interest in championing the alleged superiority of a particular design, then it's not surprising that they will continue to promote it ad nauseum. Present lots of convincing looking charts, graphs and statistics, which apparently prove their point.

Another conspiracy theory, eh? The thing is, all the optics related graphs are reproducible with raytrace. Oh, well - didn't think of it before - OSLO had to be rigged too, just to support long focus achromat. Or were it those who created diffraction calculation itself? Oh, man...

Can somebody point me to the 4" f/6.3 doublet apo mentioned in this article? I'd like to *actually* compare it to my 4" f/15 achromat.

It is a top apo doublet, comparable to TMB 130*BLEEP* in color correction (when scaled up to 130mm and f/7). It does not
quite satisfy apo requierement, but it's close. The achromat is standard Fraunhofer.

Then again, I have observed with a 4.3" f/5.95 ED doublet stopped down to 4" F/6.3 and can report that it outperforms the 4" f/15 achro in all respects, except for sensitivity to seeing.

Which proves that that particular apo was better than that particular achromat. The article considers very specific case of an apo with moderate amount of e-line correction error (0.050 wave RMS, equivalent of 1/6 wave p-v wavefront error of lower order spherical), and an achromat with an effective 1/8 wave p-v e-line error. That seems very realistic. You may have gotten luckier with your apo, and/or unlucky with the achromat...

A few years ago, I reported to another web forum my observing experiences with a 4" f/9 ED doublet and how it displayed no blue/violet haloing around bright stars.
This prompted a very well respected apo lens designer and manufacturer to suggest that the c.a. was present, but for some reason I didn't notice it. He posted spot diagrams for a 4" f/9 ED doublet which, sure enough, showed a halo.
Only trouble was, the modeling didn't match what I observed.
So, it was suggested that my vision had a problem seeing defocused light at shorter wavelengths. That didn't jibe either, since I had no difficulty observing blue/violet defocus in other refractors.
There was no conspiracy at work with the software he was using.
There was an incorrect assumption made about the lens and it's color correction. Another case of the hypothetical not matching up with reality.

A few years ago, I reported to another web forum my observing experiences with a 4" f/9 ED doublet and how it displayed no blue/violet haloing around bright stars. This prompted a very well respected apo lens designer and manufacturer to suggest that the c.a. was present, but for some reason I didn't notice it. He posted spot diagrams for a 4" f/9 ED doublet which, sure enough, showed a halo. Only trouble was, the modeling didn't match what I observed. So, it was suggested that my vision had a problem seeing defocused light at shorter wavelengths. That didn't jibe either, since I had no difficulty observing blue/violet defocus in other refractors. There was no conspiracy at work with the software he was using. There was an incorrect assumption made about the lens and it's color correction. Another case of the hypothetical not matching up with reality.

First off, switching the subject usually does not helpresolve an issue. And so doesn't not understanding (or misrepresenting?) optics and raytrace. The thing is, we do not look at the spot diagrams, and they have nothing to do with diffraction calculation.

Due to its nature (i.e. less exponential spread of energy toward farther off non-optimized wavelengths), sperochromatism does apear less colorful than secondary spectrum *at identical level of chromatic error*. Roland Christen goes as far to say that sperochrimatism is "white" (I would assume it refers to well balanced spherochromatism).

The point is, one cannot accurately judge the level of chromatic error based on the magnitude of color visible in an apo vs. achromat.

You may have gotten luckier with your apo, and/or unlucky with the achromat...

Vla

I usually try to optimize the system, when it comes to ED doublets and triplets.In the case of my apos, employing a prism diagonal (either BK7 or BAK4, depending on the objective lens characteristics) has often resulted in improved chromatic and spherical correction. Perhaps that accounts for what I've experienced.

Question for Vla--When formulating the 4" f/6.3 apo doublet, did you optimize correction for the yellow/green or blue/green?